Two weeks ago, we unveiled our model of presidential elections based on data from the past ten election cycles, which currently predicts that President Obama has a fragile advantage over his eventual opponent. We currently project that the president will win 303 electoral votes in November. But if the economic or job approval numbers slide backward even a few ticks, he loses the election by a hair.

Today we'd like to examine one rather glaring error in the model and explain why we're not going to fix it: Massachusetts stands at 74.4 percent likely to go to Obama, while all external signs dictate that it is a much safer bet for the Democrats. This is because we currently assume Mitt Romney will win the nomination, as the prediction markets suggest.

There is overwhelming empirical evidence that presidential candidates get abnormal returns in their home state. In 1984, for example, Walter Mondale still won Minnesota even though the other 49 states all went to Ronald Reagan. (Mondale also held down Washington, D.C.) So Yahoo! Labs' Patrick Hummel and I tested this theory with data from the last ten election cycles. We determined the size of that abnormal return, calibrated on those past races. This boost shifts Massachusetts from a Democratic lock to a Democratic-leaning state. It currently flips for Romney if Obama's approval rating falls to 41 percent, well before similar states.